Yes. They are safe. if you got a load of protectors, it will be better!

Can I use my fingernail to replace the stylus pen?

No, you can not, if you use you fingernail, it will hurt your screen. Stylus pen protects your screen from fingerprints, scratches, the stylus pen also improves the precision of the touch input, allowing use of smaller user interface elements!

How do I get sweat-stains out of my stylus pen?

You can use alchohol to rub and a cotton swab! That will be much better.